Generally, water pumps circulate coolants in water-cooled engines of automobiles or other machines. Such a water pump has a casing (which is also referred to a body). The casing is formed by butting a case against a cover, and an impeller is disposed in the inner space of the casing. The water pump rotates the impeller so as to draw a coolant from a radiator into the casing and cause the coolant to flow out to an engine outside the casing (Patent Document 1).
Conventionally, an example of the casing of such a water pump has a center plate (which is also referred to a partition board) so as to partition a flow path for a coolant into an upstream side (an inflow side, a radiator side) and a downstream side (an outflow side, an engine side) (Patent Document 2).
In the casing structure having such a center plate, an opening may occur in the butting surfaces between the case and the cover when an internal pressure is applied by the rotation of the impeller. For this reason, gaskets are disposed on both sides of the center plate and the two gaskets and the one center plate are sandwiched between the case and the cover so as to form sealing areas between the center plate and the case as well as between the center plate and the cover to seal a space between the case and the cover.